1. Technical Field of the Invention
This invention relates generally to radio technology and more particularly to automatic gain control within a radio receiver.
2. Description of Related Art
As is known, radio receivers may have a direct conversion architecture or a super heterodyne architecture. Direct conversion radio receivers convert a radio frequency (RF) signal directly to a baseband signal, while super heterodyne receivers convert a RF signal into an intermediate frequency (IF) signal and then convert the IF signal into a baseband signal. Both types of radio receivers include data recovery circuitry to recover data from the baseband signal.
While the architecture may be different, both types of radio receivers must include automatic gain control (AGC) circuitry to quickly and properly adjust the gain of a low noise amplifier (LNA) based on the signal strength of the received RF signal. If the gain of the LNA is not adjusted properly for a given RF signal, the radio receiver will not accurately recover the data embedded in the RF signal. For example, if the RF signal is weak, the gain of the LNA needs to be increased such that the radio receiver can detect the presence of a valid RF signal and then properly recover data from it. Conversely, if the signal strength of the RF signal is strong, the gain of the LNA needs to be decreased to avoid saturation of the radio receiver components and/or clipping of the signal.
To meet these difficult operating conditions, AGC circuitry is typically implemented in hardware, where, for a given range of received signal strength of the RF signal, the AGC hardware circuitry provides an LNA gain setting. Such hardware based AGC circuits provide quick and, under most conditions, provide proper gain settings for the LNA. However, hardware based AGC circuits are fixed and thus not easy to change or to be adjusted to accommodate certain operating conditions.
Software based AGC circuits overcome the limitations of the hardware based AGC circuits in that they can be readily changed and are adjustable to accommodate certain operating conditions. However, this flexibility comes at a price. For instance, to provide the speed comparable to the hardware based AGC circuits, a software based AGC circuit needs a dedicated processor, which, for most commercial applications, is too expensive. If a dedicated processor is not used, the software based AGC circuit shares the processor of the radio receiver, which typically makes the software based AGC circuit too slow.
Therefore, a need exists for a method and apparatus of adjusting AGC of a radio receiver that provides the advantages of both hardware and software based AGC circuits without the corresponding disadvantages.